1. Field of the Invention
The present invention relates to an in-plane switching (IPS) liquid crystal display and driving method thereof. More particularly, the present invention relates to an IPS liquid crystal display and a driving method thereof that can provide an improved opening ratio and reduced power consumption.
2. Discussion of the Related Art
Demand for a flat panel display device (of thin, small and lightweight), which can be applied to a television as well as various portable electronic devices such as a mobile phone, a personal digital assistant (PDA), and a notebook computer, has been recently increased. Various display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP), a field emission display (FED), and a vacuum fluorescent display (VFD), have been actively studied for use as a flat panel display device.
Of them, the thin, small and lightweight LCD is being spotlighted as the flat panel display device because it can provide a high-resolution image and low power consumption.
The LCD includes a liquid crystal panel for displaying an image, and a driving unit for applying a driving signal to the liquid crystal panel. The liquid crystal panel includes first and second substrates assembled to each other, and a liquid crystal layer injected between the first and second substrates.
Generally, the first substrate (or a TFT substrate) includes a plurality of gate lines arranged to be spaced apart from one another by a predetermined interval in one direction, a plurality of data lines arranged to be respectively perpendicular to the gate lines, a plurality of pixel electrodes arranged in a matrix shape in respective pixel areas defined by the gate lines and the data lines, and a plurality of TFTs turned on by signals of the gate lines to thereby transfer signals of the data lines to the respective pixel electrodes.
The second substrate (or a color filter substrate) includes a black matrix layer for intercepting light from portions other than the pixel areas, an R/G/B color filter layer for reproducing color, and a common electrode for reproducing an image. In the IPS LCD, the common electrode is formed on the first substrate.
The first and second substrates are assembled to each other. Thereafter, liquid crystal is injected between the assembled first and second substrates.
Meanwhile, the LCD is driven using the optical anisotropy and polarizability properties of liquid crystal.
The liquid crystal has directionality in a molecular arrangement because it has a thin and long structure. The molecular arrangement direction of the liquid crystal can be artificially controlled by applying an electric field to the liquid crystal.
Accordingly, if the arrangement direction of the liquid crystal molecules is changed arbitrarily, the arrangement of the liquid crystal molecules is changed and the light polarized due to the optical anisotropy is modulated arbitrarily. In this manner, the image information is expressed.
The LCD has various display modes based on the arrangement of the liquid crystal molecules. Among them, a TN LCD is widely used because it can easily display white/black color and has a fast response speed and a low driving voltage. In the TN LCD, when a voltage is applied, liquid crystal molecules aligned parallel to a substrate are aligned almost perpendicular to the substrate. Accordingly, when a voltage is applied, a viewing angle becomes narrow due to the refractive anisotropy.
In order to solve this problem, various modes having wide viewing angle characteristics have been proposed. Among them, an In-Plane Switching (IPS) LCD has been developed.
When a voltage is applied, the IPS LCD forms a horizontal electric field on the plane and aligns liquid crystal molecules on the plane, thereby improving the viewing angle characteristic.
FIG. 1 is a sectional view of a related art IPS LCD.
In FIG. 1, a pixel electrode 12 and a common electrode 13 are formed in the same plane on a lower substrate
The lower substrate 11 is bonded with an upper substrate 15 with a predetermined space therebetween. A liquid crystal layer 14 is formed between the lower substrate 11 and the upper substrate 15. The liquid crystal layer 14 operates due to a horizontal electric field formed between the pixel electrode 12 and the common electrode 13 on the lower electrode 11.
FIGS. 2A and 2B illustrate a change in the liquid crystal when a voltage is on/off in the IPS LCD.
As can be seen from FIG. 2A, a phase change of the liquid crystal 14 does not occur in the off-state, i.e., when no horizontal electric field is applied to the pixel electrode 12 or the common electrode 13. For example, the liquid crystals are twisted at 45° from a horizontal direction of the pixel electrode 12 and the common electrode 13.
As can be seen from FIG. 2B, a phase change of the liquid crystal layer occurs in the on-state, i.e., when a horizontal electric field is applied to the pixel electrode 12 or the common electrode 13. When compared with the off-state of FIG. 2A, the twist angle is about 45° and the twist direction of the liquid crystals coincides with the horizontal direction of the pixel electrode 12 and the common electrode 13.
As described above, the IPS LCD has both the pixel electrode and the common electrode on the same plane.
The IPS LCD has an advantage of a wide viewing angle. That is, when the LCD is seen from the front, the viewing angle is at 70° in up/down/right/left directions.
Also, compared with a general LCD, a fabricating process is simple and a movement of color depending on the viewing angle is slight.
However, since the common electrode and the pixel electrode are formed on the same plane, the transmission rate and the aperture ratio are reduced.
FIG. 3 is a plan view of a related art IPS LCD.
In FIG. 3, a plurality of gate lines 32 are arranged at regular intervals in one direction. A plurality of data lines 35 are arranged at regular intervals in a direction perpendicular to the gate lines 32 so as to define pixel areas P on a transparent lower substrate 31.
A common line 39 is arranged parallel to the gate lines 32 within the pixel area P. A thin film transistor T is formed at each pixel area P defined by the crossing of the gate line 32 and the data line 35.
Here, the thin film transistor T includes a gate electrode 32a protruded from the gate line 32, a gate insulating layer (not shown) formed at a front of the lower substrate 31, an active layer 34 formed on the gate insulating layer disposed at an upper portion of the gate electrode 32a, a source electrode 35a protruded from the data line 35, and a drain electrode 35b spaced apart from the source electrode 35a by a predetermined distance.
A plurality of pixel electrodes 38 are formed parallel to the data line 35 within the pixel area P. One terminal of the pixel electrode 38 is connected to the drain electrode 35b of the thin film transistor T. Also, a plurality of common electrodes 39a protruded from the common line 39 are formed within the pixel area P.
A shield electrode 36 is formed between the common electrodes 39a adjacent to the data line 35 so as to prevent distortion of an electric field.
More specifically, when a scan signal is applied to the thin film transistor T through the gate line, the thin film transistor T is turned on so that an image signal is input to the pixel electrode 38 through the data line 35. Then, an electric field is formed between the common electrode 39a and the pixel electrode 38 in a direction substantially horizontal to the substrate. The liquid crystal molecules rotate in the direction of the electric field.
However, when the image signal is input to the pixel electrode 38, the electric field is formed not only between the common electrode 39a and the pixel electrode 38 but also between the pixel electrode 38 and the data line 35.
At this time, the electric field between the pixel electrode 38 and the data line 35 distorts the entire horizontal electric field within the pixel. Therefore, the liquid crystal molecules are not arranged completely horizontal with respect to the substrate. Consequently, crosstalk occurs in a vertical direction.
The change of the electric field effects the rotation of the liquid crystals such that color tone is changed. In order to prevent this problem, a shield electrode 36 for shielding the electric field is formed between the data line 35 and the common electrode 39a and between the data line 35 and the pixel electrode 38.
For the formation of the shield electrode 36, a width of the common electrode 39a should be 10 μm or more.
Accordingly, as the width of the common electrode is widened due to the formation of the shield electrode, the aperture ratio of the pixel area is reduced.